Bug 246559

Only a partial AST is implemented.  After studying the WGSL specification, I propose:

```
Node
|- Attribute
|  |- AlignAttribute
|  |- BindingAttribute
|  |- BuiltinAttribute
|  |- ConstAttribute
|  |- GroupAttribute
|  |- IdAttribute
|  |- InterpolateAttribute
|  |- InvariantAttribute
|  |- LocationAttribute
|  |- SizeAttribute
|  |- StageAttribute
|  |- WorkgroupSizeAttribute
|- Directive
|- Expression
|  |- BinaryExpression
|  |- BitcastExpression
|  |- IndexAccessExpression
|  |- CallExpression
|  |- IdentiferExpression
|  |- LiteralExpression
|  |  |- BoolLiteral
|  |  |- FloatLiteral
|  |  |- IntegerLiteral
|  |- FieldAccessExpression
|  |- UnaryExpression
|- Function
|- ShaderModule
|- Statement
|  |- AssignmentStatement
|  |- BreakStatement
|  |- CallStatement
|  |- CompoundAssignmentStatement
|  |- CompoundStatement
|  |- ContinueStatement
|  |- DecrementStatement
|  |- DiscardStatement
|  |- ForStatement
|  |- IfStatement
|  |- IncrementStatement
|  |- LoopStatement
|  |- ReturnStatement
|  |- StaticAssertStatement
|  |- SwitchStatement
|  |- VariableStatement
|  |- WhileStatement
|- Structure
|- StructureMember
|- TypeAlias
|- TypeName
|  |- ArrayTypeName
|  |- NamedTypeName
|  |- ParameterizedTypeName
|- Value
|  |- ConstantValue
|  |- OverrideValue
|  |- LetValue
|  |- ParameterValue
|- Variable
|- VariableQualifier

```